An MPLS Label Switched Path (LSP) is a uni-directional “tunnel” originating at one label edge router (LER), passing through zero or more transit label switch routers (LSRs), and terminating at an egress LER. The tunnel may be thought of as a “virtual circuit” connecting one LER to another LER via an OSI layer 2 physical connection. To set up an LSP, an Internet Protocol (IP) signaling protocol such as a Resource ReSerVation Protocol (RSVP) or Label Distribution Protocol (LDP) is commonly used.
LSPs are generally set up by issuing a request on an ingress LER via a command line interface (CLI) command or network management tool running somewhere in the network which sends commands to the ingress LER to create the LSP. The LSP can be configured to take a specific, non-specific or normal IP routing path through the network to reach the egress LER. It can also be configured optionally to provide specific bandwidth or quality-of-service (QoS) characteristics such as low-latency (i.e., minimally senescent packets), low-jitter and/or priority. LSPs provide an increasingly popular way to create virtual circuits needed to seamlessly connect geographically distributed users with high bandwidth, low jitter, virtual circuitry.
LSP tunnels are more useful if made to appear to be bi-directional such that traffic can flow in both directions between the endpoints. To create a bi-directional virtual circuit, two LSPs must be established—one in each direction originating at the two endpoints. This kind of tunnel causes additional administrative difficulty in that two separate LSPs have to be setup. Furthermore, it is desirable to have the two parts of the bi-directional virtual circuit pass along the same physical path so that the chance of communication interruption is minimal. This requirement adds to the burden of the administrator(s) charged with setting up the LSPs since the first leg of the bi-directional LSP, a uni-directional LSP, may take a particular path which is not known until the LSP is fully set up. The second leg of the bi-directional LSP, that is the complementary uni-directional LSP originating at the egress LER of the first LSP and terminating at the ingress LER of the first LSP, ideally needs to follow the same path and must therefore be explicitly specified.
This kind of return LSP can be established manually; that is, with human intervention and judgment. Some of the required information is only available at the egress LER of the forward LSP, thus time may also be required to obtain the required information from the possibly remote physical location. What is needed therefore is a procedure to automatically establish a return LSP that traverses the same path between two endpoints as a forward LSP.